Corrosion resistant boiler tube for chemical recovery vapor generating unit

ABSTRACT

A corrosion resistant boiler tube for chemical recovery vapor generating units, in which a boiler tube of carbon steel or low alloy chromium-molybdenum (Cr-Mo) steel has weld deposited on its exterior surface a coating of high chromium steel containing 1326 percent chromium and 0.5 - 1.5 percent columbium.

United States Patent Hayashi et a1.

Jan. 23, 1973 CORROSION RESISTANT BOILER TUBE FOR CHEMICAL RECOVERY VAPOR GENERATING UNIT Inventors: Tsuneto Hayashi; Hidejiro Kinoshita; Koji Iwahashi, all of Nagasaki, Japan Assignee: Messrs. Mitsubishi J ukogyo Kabushiki Kaisha, Tokyo, Japan Filed: Jan. 8, 1970 Appl. No.: 3,573

Related U.S. Application Data Continuation of Ser. No. 671,075, Sept, 27, 1967, abandoned.

Foreign Application Priority Data Sept. 29, 1966 Japan ..4l/9l214 U.S. Cl. ..138/l45, 29/196.l,138/177 Int. Cl. F161 9/14, F161 9/02 Field of Search ..138/l42,145,177,127,134; 29/1961, 196.6, 198; 219/76 [56] References Cited UNITED STATES PATENTS 2,963,129 12/1960 Eberle ..29/196.1 X 3,288,575 11/1966 Bystram ..29/l96.l

OTHER PUBLICATlONS National Petroleum News," pp. R132, Rl34-138, Alloy Steels For Modern Still Tubes by R. L. Wilson, Issue of March 23, 1938.

Primary Examiner-Herbert F. Ross Attorney-McGlew and Toren [57] ABSTRACT A corrosion resistant boiler tube for chemical recovery vapor generating units, in which a boiler tube of carbon steel or low alloy chromium-molybdenum [Cr-Mo] steel has weld deposited on its exterior surface a coating of high chromium steel containing 13-26 percent chromium and 0.5 1.5 percent columbium.

6 Claims, 9 Drawing Figures PATENTEDJAHZBIQH 3.712.317

SHEETIUFS FIG. I

O (I n: O 20.06-

x 2 o l o 5 IO I5 3o Cr CONTENT(%) H6. 7 INVENTOR$ TSUNETO HhYnsm HIDEJIRa KINDS/41TH K01: 'mnuns'm M MM {and PATENTEUJAH 23 ms SHEET 2 OF 5 PAIENTED JAR 23 1915 SHEET 51 0F 5 CORROSION RESISTANT BOILER TUBE FOR CHEMICAL RECOVERY VAPOR GENERATING UNIT This is a continuation of application Ser. No. 671,075, filed Sept. 27, 1967, now abandoned.

BACKGROUND OF THE INVENTION In the production of paper pulp, useful chemicals are recovered and steam power requirements are supplied, at least in part, by burning the waste liquor, used for digestion of the pulp, in a chemical recovery vapor generating unit. For example, such a unit may be used to recover soda smelt as well as to generate steam for the production of electric current and for the steam requirements of the pulp production plant. In the chemical recovery unit or boiler, the organic ingredients of the waste liquor, such as lignin and sugars, are burned. During combustion, the inorganic components, such as principally sodium carbonate and sodium sulfate, are melted and the resulting soda smelt is recovered at the base of the combustion chamber of the recovery unit.

Recently, plants for the production of pulp have been larger size and, because of this larger size, the chemical recovery units have been designed to operate at higher temperatures and under higher pressures. When the recovery unit is operated under the more severe conditions of higher temperature in the walls of the boiler tubes located in the combustion chamber, in the super heater, etc., the temperature can reach 350C or more. Under these conditions, severe difficulties are encountered in that the tubes are severely corroded due to the attack by soda smelt.

With boiler tubes of carbon steel or low alloy Cr Mo steel, corrosion due to the attack by soda smelt occurs in those tubes which are located in the combustion chamber at and near to the inlet for the introduction of primary air, which is the location of highest temperature. Such corrosion also occurs preferentially in tubes located in the lower bent portion of the super heater. In some cases the severe corrosion, the rate of corrosion of the boiler tubes is of the order of 1 mm. per year in a unit operating under pressure of 100 kg/cm. As the wall thickness of a boiler tube in the combustion chamber is normally from 5 to 6 mm., the useful life of boiler tubes in the combustion chamber is usually about 4 to 5 years.

From the foregoing, it will be noted that corrosion usually occurs in tubes positioned in limited zones of the chemical recovery unit. Accordingly, if the tubes used in such limited zones can be protected by using a material having good resistance to attack by soda smelt, the durable life of the tubes may be increased considerably and it would be possible to construct a chemical recovery unit having a greater overall efficiency. Various methods have been proposed to pro tect the walls of the tubes located in such limited zones. In one method, a number of studs are welded onto the tube wall and the interspaces between the studs are filled with an anti-corrosive material resistant to the attack of soda smelt. In another method or process, an anti-corrosive material is sprayed onto the tube walls. In still a further method or process which has been proposed, a protecting material of stainless steel is secured to the tube wall.

However, the first process mentioned above has the drawback that the studs corroded and reduced in length in a relatively short time, and to the extent that the anti-corrosive material applied in the interspaces is liable to fall out. In the second process mentioned above, wherein anti-corrosive material is sprayed onto the tube walls, this material is adhered only mechanically to the tube walls, and there is therefore the disadvantage that the anti-corrosive material has a tendency to strip off from the tube walls. With the third process mentioned above, there is the hazard that the tube can be cracked owing to the difference in thermal expansions between the protecting material of stainless steel and the base material of the tube, if the protecting material is inadequately secured to the base material of the tube. This third method has the further disadvantage, among others, that the heat efficiency of the recovery unit is reduced.

SUMMARY OF THE INVENTION are avoided. Thus, in accordance with the invention, an

inexpensive high chromium steel containing chromium and columbium is weld deposited on the exterior surface of the wall of a boiler tube of carbon steel or low alloy chromium-molybdenum steel. The weld deposited material not only has physical properties similar to those of the base material of the tube, but also has mechanical properties superior to those of the base material. Thus, the weld deposited material has a substantially higher resistance to attack by soda smelt than does the base material, and moreover can be readily welded to the base material.

In particular, the material weld deposited on a base material of carbon steel or low alloy chromium-molybdenum steel comprises a high chromium steel contain- I ing 13 26 percent Cr and 0.5 1.5 percent Cb. A boiler tube in accordance with the present invention has a remarkably high resistance to corrosion, due to the particular coating weld deposited on the surface of the base material. With the boiler tube of the present invention, the anti-corrosive deposited layer can neither drop off nor strip off the base material, because the weld deposited material has weldability so good that the weld deposited layer can be adhered strongly and closely to the base material of the boiler tube. This is furthermore due to the fact that the weld deposited material has physical properties very similar to those of the base material of the boiler tube. An additional advantage is that the weld deposited material, in the case of a boiler tube embodying the present invention, is less expensive than Cr-Ni stainless steel. Consequently, the overall expense of the boiler tube is reduced while the boiler tube still has a long useful life since it exhibits a combination of the above-mentioned advantages and also has a high resistance to corrosion due to soda smelt.

Boiler tubes embodying the present invention may be formed by weld depositing high chromium steel, containing a specific proportion of columbium, onto the base material by a hand welding method or by a submerged arc welding method. The weld depositing of the protective material onto the base material is effected at those portions of the surface of the basic tube where corrosion is likely to occur due to attack by soda smelt. In order to provide an appropriately long life for the tube, it is necessary that the thickness of the weld deposited layer should be 1 mm or more. In practice, it is preferred, from the standpoint of economics, that the thickness of the weld deposited layer be up to 5 mm.

Each tube may have a coating weld deposited thereon while it is still in the form of a single tube, the distortion of the tube may then be corrected by suitable measures, a number of the coated tubes may thereafter be assembled in a panel, and the assembly may then be annealed to eliminate stress in the assembly. Alternatively, the weld depositing of the coating may be effected after a group of tubes have been assembled into a panel.

The high chromium steel used as the weld deposited protective coating in the tube of the present invention should contain the specific portions of chromium and columbium, because the percentages of chromium and columbium in the high Cr steel are critical as will be made apparent hereinafter.

Accordingly, an object of the present invention is to provide an improved corrosion resistant tube for use in waste recovery vapor generating units.

Another object of the invention is to provide such a tube comprising a base material having a high Cr steel weld deposited on its exterior surface.

A further object of the invention is to provide such a tube in which the base material is carbon steel or low alloy chromium-molybdenum steel and the protective coating weld deposited thereon is a high Cr steel including a specific proportion of columbium.

Still another object of the invention is to provide such a tube formed of carbon steel or low allow chromium-molybdenum and having weld deposited on its outer surface a coating of high chromium steel containing 13 26 percent Cr and 0.5 1.5 percent Cb.

A further object of the invention is to provide such a boiler tube which is particularly resistant to corrosion due to attack by soda smelt.

Yet, another object of the invention is to provide such a tube including a base material having weld deposited thereon a corrosion resistant material, and in which the corrosion resistant material has physical properties similar to those of the base material but corrosion resisting properties very substantially in excess of those of the base material.

BRIEF DESCRIPTION OF THE DRAWINGS For an understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawings. In the drawings:

FIG. 1 is a graph which shows a relation between the chromium content of chromium steel and the rate of corrosion of the chromium steel due to the attack of soda smelt;

FIGS. 2a and 2b are microphotographic views of the deposition welded layer of 18 Cr l Cb steel on the boiler tube of the present invention and of the weld deposited layer of 18 Cr steel;

FIG. 3 is a graph which shows the results of tests determining the rates of corrosion of various steel materials weld deposited on the surface of the boiler DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. l,which graphically illustrates the relation between the chromium content and the rate of corrosion due to attack by soda smelt, it may be seen that the anti-corrosive properties of a chromium steel increase remarkably when the chromium content is 13 percent or greater. It appears that this high anti-corrosive property is due to the passive state which will be developed in the surface of the chromium steel. Accordingly, the lower limit of chromium content should be 13 percent. Further increase of the chromium content beyond 26 percent is not economical, and it does not result in any substantial further improvement in the anti-corrosive properties. Sometimes, it actually can produce coarser grains and deteriorate the mechanical properties of the chromium steel. Consequently, the upper limit of the chromium content, in accordance with the present invention, should be 26 percent.

The presence of columbium makes the grains of the chromium steel fmer,'and therefore results in improving the mechanical properties of the chromium steel and particularly the elongation and contraction of area of the chromium steel tube. The proportion of columbium in the high chromium steel should be within the range of 0.5 to 1.5 percent in accordance with the present invention, because the content of columbium less than 0.5 percent is not effective in making the grains of the chromium steel finer and because a content of columbium more than 1.5 percent is neither economical nor can it produce any further improvement in the mechanical properties of the chromium steel.

Referring to FIG. 2a, which is a microphotographic view of an 18 Cr l Cb steel as the weld deposited material in accordance with the invention, and referring to FIG. 2b, which is a microphotographic view of an 18 Cr steel used as a reference material, it will be seen that the grains in the columbium-containing chromium steel are very much finer than those in the reference chromium steel. These microphotographic views are magnified times. The boiler tube embodying the present invention will now be illustrated with reference to some embodiments thereof.

The corrosion-resistance, physical properties i and mechanical properties of the boiler tube in accordance with the present invention and made by weld depositing a weld 2 of 18 Cr l Cb high chromium steel on the surface of a base material, comprising a steel tube 1 of carbon steel STB 35 were tested, in comparison with the same properties of the base material of the boiler sistance to smelt corrosion than does the 18 Cr 1 Cb steel, but its corrosion resistance may still be sufficient for certain uses within the scope of the present invention. The coating of 26 Cr l Cb steel has a further tube of Carbon Stee STB 35, of another boiler tube of 9 5 improved resistance to smelt corrosion as compared to Cr 1 Mo steel, and of a prior art protecting material that of the 18 Cr l Cb steel. comprising 18 Cr 8 Ni stainless steel. The composi- The thermal expansion coefficient and the thermal tions of the carbon steel STB 35 and the 18 Cr l Cb conductivity of conventional carbon steel STB 35, as steel, used in the test, are tabulated in Table 1 below: 0 well as those of 18 Cr 1 Cb steel, 13 Cr 1 Cb steel TABLE 1 1 and 26 Cr l Cb steel, used as the weld deposit coating in accordance with the present invention, are tabu- Ingrediem in per cent lated in Table 2 below:

TABLE 2 Steel C Mn Si P S Cr Cb Carbon 0080.30 0.10 less 15 Thermal ex- Thermal steel than than pansion coefficconductivity STB 35 0.18060 0.35 0.035 0.035 Steel Temperature cient X 10' Cal/cm/S/C 18 Crin C 1 Cb 0070.31 0.56 0.022 0.009 17.7 0.9 Carbon 100 11.9 0.122 steel steel 300 13.0 0.1 10 STB 35 500 14.2 0.094

The curves illustrated graphically in FIG. 3 show the 1 Cb 100 10.0 results of tests determining the rate of corrosion of the steel 28?, :58 382; different steel materials positioned in a solidified soda 13 Cr smelt at different temperatures. Curve 1 represents the i s: $83 3'? results obtained with a tube formed of carbon steel STB 500 11:5 0.069

' 26 Cr 35, Curve 11 represents the results obtamed with a tube 1 Cb 100 100 0052 of 9 Cr 1 Mo steel, Curve Ill represents the results Steel 300 11,0 obtained with a protecting material of 18 Cr 8 Ni 500 stainless steel, and Curve 1V represents the results ob- Table 2 Shows that the average coeffjlclem of thermal tained with a tube formed of carbon steel STB 35 hav- 30 expanslo of Z 18 1 Cb steel '2 a tempeliature ing weld deposited on its outer surface 18 Cr l Cb ofhloohto 500 C g 82 stilghtly steel in accordance with the present invention. As will Z er 2 Va ue f o t be seen from a comparison of these curves, the re- 9 stee t 6 Same range 0 tempera a Since the weld depos1ted layer of 18 Cr 1 Cb steel on sistance to smelt corrosion of a tube havmg a coatmg of the surface of the boiler tube of the present invention, 18 Cr l Cb weld depos1ted thereon, in accordance wh1ch contacts w1th the gases at higher temperature with the invention, is 12 t1mes higher than that of the h ff f h l l h h tube of carbon steel STB 35 and that of the tube of 9 as a C08 101cm 0 erma expanslqn -smal er t an t at C 1 M t 1 t l l d t t H of the base material of the tube, it is clear that the t r fth o d f 3: 5 3 We y mo era 6 es empera' boiler tube of the invention can resist heat-stress with h f b 40 safety. Although the 13 Cr 1 Cb steel has a slightly b 1 eregce t a g lower coefficient of thermal expansion that the 18 Cr e "3 t e F P" c 1 cb steel, the boiler tube with this 13 Cr 1 Cb corroslon reslstance reference matenals grefater as steel weld deposited thereon can also resist heat-stress the test temperature 15 increased. The corrosion rewith Safety It may be appreciated that the 26 Cr t slstance a 9 tube formed f accmdance the Cb steel has a coefficient of thermal expansion smaller present invention is not much different from that of a than that f the 13 Cr 1 Ch steel reference boiler tube of 18 Cr 8 N1 sta1nless steel at Thermal conductivity f the 13 1 steel i test temperatures in Vicinity of C, but at higher somewhat lower than that of the carbon steel STB 35, temperatures, the resistance of the reference boiler b hi d t produce t bl in h ration of tube of 18 Cr 8 Nhstalnless Stee 15 sub n y the boiler tube of the present invention. Thermal conlower than that of a bo1ler tube of the present mvend u i f h 13 C 1 Ch steel d of h 26 C t1on. Furthermore, it has been found that a 13 Cr l l Cb steel is substantially of the same order as that of Cb steel, which may be used as the weld deposited corthe 18 Cr l Cb steel and is better than that of 18 Cr rosion resistance coating within the scope of the 8 Ni stainless steel, through the table does now show present invention, shows a more or less lower rethis.

TABLE 3 Contrae- True Treatment alter Yield Tensile Elongation of break- Hard deposltlonpoint, strength, tion, area, ing load, ness Steel welding, C. kg./mm.' kg./mm. percent percent percent Hv Carbon steel, STB 35-. Annealed at 900... 26.0 43. 0 35.0 67. 5 8 27. a 44. 2 34. e 66. 0 132 No treatment. as. 5 55.1 20. 0 62. 0 0s. 0 180 18 (Jr-1 Cl) S1100] 31). 5 5B. 3 28. (l M. 6 71]. 2 182 Annealed at 628": 40. 4 58. 3 21). ll 58. l 85. 5 175 18 (r steel N0 trl utmcnt l l l 6 {Annealed at 152.8 70. 2 3. 2 14.0 70. 2 2x0 13 (Jr-1C1) steel..." Annealed at. 025 48. 8 03. 3 37. l) 4 .507 26 CH Cb steel d0 40.0 00. 4 26.1)

In Table 3, there are shown the mechanical properties of the conventional carbon steeel STB 35 as well as of the 18 Cr 1 Cb steel, the 13 Cr l Cb steel and the 26 Cr 1 Cb steel which are used as the weld deposited coating material for the boiler tube according to the present invention. The mechanical properties shown in Table 3 have been determined'at normal termperature.

The graph of FIG. 4 includes curves which represent the variations of tensile strength of the different steels at higher temperatures, as well as curves which represent the variations of yield point of the steels at higher temperatures.

The upper group of curves in the graph of FIG. 5 represents the variation of sinking-workability of the steels at higher temperatures, and the lower group of curves represents the variation of elongation of the steels at higher temperature.

The 18 Cr 1 Cb steel exhibits higher yield points and higher tensile strengths, but somewhat lower elongation and lower contraction of area, than the carbon steel STB 35, though it has a strength and toughness sufficient to be used as the weld deposited coating material according to the present invention. Both the 13 Cr l Cb steel and the 26 Cr l Cb steel similarly have mechanical properties sufficient for use as the weld deposited material according to the present invention.

The 18 chromium steel, however, clearly cannot be used as the weld deposited coating material, as it has a very much lower elongation and contraction of area than does the 18 Cr 1 Ch steel. It is apparent that the 18 Cr 1 Cb steel exhibits a lower yield point than does the carbon steel STB at higher temperatures, and that the 18 Cr l Cb steel, as well as the 13 Cr 1 Ch steel and 26 Cr 1 Cb steel, show higher tensile strengths than does the carbon steel STB 35 at higher temperatures. It is seen that the 18 Cr l Cb steel has a more or less lowe contraction of area than the carbon steel STB 35 and that the 18 Cr l Cb steel as well as the 13 Cr l Cb steel and 26 Cr 1 Cb steel exhibit somewhat lower elongations than the carbon steel STB 35. However, these columbium steels still retain a toughness sufficient to be used as the weld deposited material with advantage.

When tubes embodying the present invention were used in an actually operating chemical recovery vapor generating unit, for recovering soda smelt, it was demonstrated practically that the durable life of the unit could be prolonged. Thus, when it was found that boiler tubes in the bottom portion of an operating unit for recovery of soda smelt had been corroded, and ex hibited a dangerous state, some such corroded boiler tubes were repaired by weld depositing thereon a coating rod using a welding rod of mild steel, and others of the corroded boiler tubes were repaired by weld depositing thereon a coating using a welding rod of 18 Cr l Cb steel. After the repaired boiler tubes were reinstalled and the unit operated for a further period of about ten months, the tubes were removed from the v unit and examined for surface conditions. It was found that those tubes having a coating of mild steel weld deposited thereon had been greatly corroded, as shown by the photographic view in FIG. 6A. On the other hand, those tubes coated with the weld deposited layer of 18 Cr 1 Cb steel did not have any substantial corrosion, as shown by the photographic view of FIG. 6B.

As previously mentioned, conventional tubes of carbon steel or low alloy CrMo steel have usually been corroded, after only three to four years, to the extent that the wall thickness of the tubes has been reduced below the lower limit of the safety requirement of the vapor generator, and it is usual to replace conventional tubes, made of carbon steel or low alloy Cr-Mo steel, by new ones after three to four years. However, boiler tubes formed in accordance with the present invention have a durable life of several times to ten times that of conventional boiler tubes, because the high chromium steel containing 13 26 percent Cr and 0.5 1.5 percent Cb deposited as a surface layer on the boiler tube is highly resistant to attack by soda smelt.

FIG. 7 illustrates tubes 1, provided with a weld coat ing 2 in accordance with the invention, as arranged to form at least part of the wall of the furnace of a vapor generator, such as a chemical recovery vapor generator unit for recovering soda smelt.

The high chromium steel, containing 13 26 percent Cr and 0.5 1.5 percent Cb, weld deposited as the surface layer of boiler tubes in accordance with the invention exhibits physical properties very similar to those of the carbon steel which is the base material of the tube, and moreover it has stronger mechanical properties than does the carbon steel. These properties of the chromium steel containing columbium, and having high resistance to corrosion, provide a tube formed in accordance with the invention with a toughness so high I that a recovery unit can be operated at higher temperatures and pressures with greater efficiency than prior recovery units. Furthermore, when using a tube in accordance with the invention, it is not necessary to use an expensive Cr-Ni stainless steel. Accordingly the tube in accordance with the invention has very substantial advantages from the economic standpoint.

We claim:

1. For use in a high temperature, high pressure chemical recovery vapor generating unit, designed for the recovery of soda smelt, and in which boiler tubes are subject to corrosion due to attack by the soda smelt, a boiler tube comprising a steel body; and a weld of high chromium, fine-grained steel having physical properties and a coefficient of thermal expansion substantially equal to those of said steel body, mechanical properties superior to those of said steel body, and a resistance to attack by soda smelt substantially higher than that of said steel body, said weld deposited on the exterior surface of said body, said high chromium steel consisting only of 13 26 percent chromium and 0.5 1.5 percent columbium, with the balance iron containing the usual impurities.

2. A boiler tube, as claimed in claim 1, in which said chromium steel consists of 18% Cr and 1% Cb, with the balance Fe and the usual impurities.

3. A boiler tube, as claimed in claim 1, inwhich the high chromium steel consists of 13% Cr and 1% Cb,

5. A boiler tube, a claimed in claim 1, in which the 6. A boiler tube, as claimed in claim 1, in which said steel of said body is selected from the group consisting body is formed of carbon steel. of carbon steel and low alloy Cr-Mo steels. 

2. A boiler tube, as claimed in claim 1, in which said chromium steel consists of 18% Cr and 1% Cb, with the balance Fe and the usual impurities.
 3. A boiler tube, as claimed in claim 1, inwhich the high chromium steel consists of 13% Cr and 1% Cb, with the balance Fe and the usual impurities.
 4. A boiler tube, as claimed in claim 1, in which the high chromium steel consists of 26% Cr and 1% Cb, with the balance Fe and the usual impurities.
 5. A boiler tube, as claimed in claim 1, in which the steel of said body is selected from the group consisting of carbon sTeel and low alloy Cr-Mo steels.
 6. A boiler tube, as claimed in claim 1, in which said body is formed of carbon steel. 